1. Field of the Invention
The present invention relates generally to position location, and specifically to a receiver that enables the reception of television and GPS signals for the purpose of position determination.
2. Description of the Related Art
The Global Positioning System (GPS) technology has revolutionized the field of navigation and position location. Initially devised in 1974, the GPS is based on a constellation of 24 on-orbit satellites in 12 hour circular, inclined orbits. Each satellite transmits a known pseudo-noise signal synchronized to an on-board precision atomic clock. The transmitted pseudo-noise signals can be precisely tracked by a receiver at an unknown location to determine pseudo-ranges. By tracking four or more satellites, one can determine precise position in three dimensions in real time, world-wide. More details on the operation of GPS are provided in (1) B. W. Parkinson and J. J. Spilker, Jr., Global Positioning System-Theory and Applications, Volumes I and II, AIAA, Washington, D.C. 1996, and (2) J. Spilker, Jr., Digital Communications by Satellite, Prentice-Hall, Englewood Cliffs, N.J., 1977, 1995, which are incorporated by reference herein in their entirety.
However, the effectiveness of GPS is limited in some situations. Because the GPS signals are transmitted at relatively low power levels (less than 100 watts) and over great distances, the received signal strength is relatively weak (on the order of −160 dBW as received by an omni-directional antenna). Thus the signal is marginally useful or not useful at all in the presence of line-of-sight blockage or while the receiver is inside a building.
This limitation or shortcoming of GPS technology can be overcome or ameliorated by employing position location technologies based on receiving broadcast television signals. High power, high bandwidth, and superior geometries make various broadcast television signals ideal candidates for augmenting or improving position location where the GPS technology fails. For example, various digital television (DTV) broadcast signals contain embedded synchronization codes which can be used for position determination purposes. The ATSC standard in the United States, the DVB standard in Europe, and the ISDB-T standard in Japan all employ an embedded synchronization code which is used to probe the transmission channel and mitigate the effects of multipath in a digital TV receiver. In order to be effective for channel modeling and multipath mitigation, these synchronization codes have wide bandwidths, narrow time autocorrelation functions, and high power levels. These features make these synchronization codes ideal for positioning, in particular for indoors positioning where multipath effects are severe and GPS signals may not penetrate. In addition, the Ghost-Canceling Reference (GCR) signals embedded in analog television broadcasts can be used for precise ranging. In recent years, analog television broadcasts have started to insert into their broadcasts a synchronization code termed the Ghost-Canceling Reference, which is used for multipath mitigation on analog signals in TV receivers that digitize the signal. High power characteristics and wide availability of GCR signals make them suitable for position location. Other test signals inserted in the analog broadcasts, such as the multiburst signal, may also be used for position determination. In the GPS system, autocorrelation of a known Pseudo-Random-Number (PRN) sequence signal (i.e., a pseudo-noise signal) is used to determine pseudo-ranges. Other test signals inserted in the analog broadcasts, such as the multiburst signal, may also be used for positioning.
There has even been a proposed system using conventional analog National Television System Committee (NTSC) television signals to determine position. This proposal is found in a U.S. Patent entitled “Location Determination System And Method Using Television Broadcast Signals,” U.S. Pat. No. 5,510,801, issued Apr. 23, 1996. However, the technique described the use of the horizontal and vertical synchronization pulses which were intended only for relatively crude synchronization of the TV set sweep circuitry, and cannot achieve the level of positioning accuracy or reliability of the disclosed location technology. Further, in 2006 the Federal Communication Commission (FCC) will consider turning off NTSC transmitters and reassigning that valuable spectrum so that it can be auctioned for other purposes deemed more valuable.
A strong emphasis is being placed on the mobile user for the type of services that DTV can provide. For example, efforts are ongoing in Korea and Japan to generate cellular handsets which include television tuners for the purpose of receiving television on the mobile device as well as data conveyed using the DTV channel. A correct design of the receiver architecture, as described in this disclosure will enable data reception, as well as location using TV signals in combination with GPS.
One problem with conventional position determination technology relates to the capacity and processing circuitry of the user device. The computations to ultimately determine position location, particularly when using both GPS and television signals, can be complex to say the least. In many instances, the position location technology is placed in a compact mobile device. A typical mobile device has finite processing power, limited analog front end sophistication, a relatively limited power source and restricted memory capacity. In the case of a handheld receiver or telephone with receiver functionality for position location, a limited amount of real estate may exist on the printed circuit board(s) to implement sophisticated circuitry for managing receiver functions. Additionally, the more circuitry designed onto the card, generally the greater the battery consumption, which may be especially undesirable for mobile devices such as transceivers or cellular telephones.
These problems and limitations that exist with mobile devices are exacerbated in light of the objective that the receiver perform position computations in as close to real time as possible. This may be especially true where the user device is in motion, and it is desirable to obtain as high a quality position measurement as possible. An effective method and system is needed to perform the required computations for position location as expediently as possible and using minimal computational requirements given the limitations on size, memory capacity, and processing power on many of these user devices.
To this end, one possible approach is to employ a “software” based receiver device which uses, in addition to an analog front end for receiving, filtering, amplifying and digitizing the broadcast television signal, a processor such as a DSP or microprocessor to sample a sequence of the downconverted broadcast signal, and then to process the sample by executing necessary routines that manipulate the digitized signal. Such a software implementation, designed properly, can have the advantages of mitigating multipath interference and reducing the complexity of circuitry on the integrated circuits that are embedded within the mobile device. However, given the critical timing requirements and relative sophistication of a digital broadcast signal, even a solution using a processor or DSP to perform primary correlation functions can produce inaccurate results, and without any appreciable decrease in computational requirements. Additionally, the traditional correlation process for identifying time offsets between transmissions of a known code sequence and its arrival at a mobile receiver can add considerable complexity into the mix. In particular, for each signal segment interval (e.g., a single packet of information in a television broadcast signal), a separate correlation operation may need to be performed to extract timing information from each segment. As the number of intervals increase, the number of required correlation operations likewise increases, which taxes the processing unit of the receiver and limits the capability of smaller mobile devices to produce position-related information (either for itself or a location server or other computing device) in near real time as is highly desirable in such mobile applications.
As the demand for progressively smaller and more compact mobile devices continues, it becomes more desirable and advantageous to implement a processor-based solution that eliminates much of the hardware otherwise necessary to process the signal and extract critical timing information for purposes of position location. Accordingly, a need exists in the art to provide a receiver device capable of using one or more processor(s) in a manner that reduces computational load and results in accurate position determination in near real time without consuming unnecessary power or overtaxing the memory capacity of the device.